We discuss the structure and formation mechanism of ZnSe/GaAs(100) interfaces as probed by reflectance difference spectroscopy (RDS). First we describe a simple analytic procedure that separates the surface and interface contributions in the RD spectra obtained for a heterostructure. The procedure opens up the new possibilities of RDS as an in situ interface probe. We have applied this technique to characterize the ZnSe/GaAs interfaces prepared on various GaAs(100) surfaces. The interface-anisotropy spectra thus obtained and the cross-sectional transmission electron microscopy, show clearly that the chemical composition of the interfacial layer can be changed by controlling the reconstruction and the pregrowth Zn and Se treatments of the initial GaAs surface.